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358                             CHEMICAL ENGINEERING
until the salt crystals separate, and glucose or maltose solutions are concentrated until they reach the consistency of syrup. The main object is the recovery of the solids in a more or less concentrated form, and the solvent is wasted in practically all cases.
The various methods of evaporation may be divided into the following classes:
First: By direct contact between heating medium and liquid: (a) Evaporation by waste gases and heated air. (&) Evaporation by the rays of the sun (solar evaporation).
Second: By indirect contact between heating medium and liquid through metallic surfaces:
(a) Evaporation by direct heat produced by coal, gas or oil.
(6) Evaporation by indirect heat conveyed by steam, hot water, oil or electricity.
All the methods mentioned above are used in the chemical and allied industries for the concentration of various liquors, but there is no doubt that the heating with steam gives the best results, and most of all, the highest efficiency. In practically all cases, water is the solvent, and as early as 1829 Pecqueur found that it was possible to use the vapor produced in. one evaporator as a heating medium in the second apparatus working at a lower pressure and temperature. The vapor produced in this body can be used again in the next evaporator, and so on as long as there is sufficient temperature difference between vapor and liquid to cause boiling of the liquid. A pound of coal with about 13,000 B.t.u. will produce in a steam boiler about 8 Ib. of steam with 9,100 B.t.u., an efficiency of 70 per cent. This amount of steam, however, will evaporate again about 7 Ib. of steam in one evaporator (single-effect), 14 Ib. in two evaporators (double-effect), 21 Ib. in three evaporators (triple-effect), and so on. In a combination of a boiler and triple-effect, 1 Ib. of coal of 13,000 B.t.u. would therefore produce 29 Ib. of steam of about 335000 B.t.u., an efficiency of over 250 per cent. This is a very important fact, and the low cost of two of our most essential staple articles of food, sugar and salt, are due to the high efficiency of multiple-effect evaporators; also soda pulp could not be manufactured in competition with sulphite pulp if the soda in the weak wash waters could not be recovered economically by multiple-effect evaporation.
Waste Gases and Heated Air.—This method consists of bringing the hot air or gases in intimate contact with the more or less finely divided liquid. For instance, the liquor is placed into large shallow pans and the furnace waste gases are conducted over the top of the pan. Even under favorable conditions, the contact surface is small and the evaporating capacity is low, i.e., from 2 to 3 Ib. of water per hour per square foot, depending on the temperature of the gases. A much larger contact surface and better efficiency is obtained in the so-called Carlson Evaporator, which is used in the sulphate-pulp mills and where a large surface is exposed to the waste gases by leading these gases through a series of revolving discs which are partly submerged in the liquid. To the same class belong the spray evaporators, where the liquor is divided into fine particles by means of a horizontal disc rotating at a high speed, or by discharging it under high pressure from spray nozzles. This method is used mostly for drying purposes, and will be described in more detail in the next sub-section: "Drying." In all cases, the evaporation is effected by lowering the temperature and increasing the humidity of the waste gases or hot air; consequently, large volumes are required and the efficiency is low.
Solar Evaporation.—Nature has given us in the sun's rays a practically inexhaustible supply of heat, but it can only be utilized to advantage in warm dry